The present invention relates to a wideband radiocommunication system such as CDMA (Code Division Multiple Access), and more specifically, a radio-communication device having an up-link power control function and a base station, which are used in the wideband communication system, and a method of controlling the up-link power.
In recent years, a radiocommunication system has begun to be put into practice which is based on the CDMA technique in which, by spreading an information-bearing transmit signal with a code to produce a wideband spread signal, channels can be assigned on a code-by-code basis. In 2001, a W-CDMA system will be put into practice which allows more information to be transmitted by making the spread spectrum wider than at present. However, the frequency band presently assigned to up-link signals in the W-CDMA in Japan is 1920 to 1980 MHz, which is contiguous to the frequency band, about 1893 to about 1920 MHz, assigned to the PHS (personal handyphone system) system which have already captured a large number of subscribers.
A power amplifier used in the transmitter of a radiocommunication device needs to be high in power efficiency even at the time of transmission at maximum power in order to make a lifetime of a battery as long as possible. However, this means that the power amplifier is used at an operating point close to saturation. With amplitude modulation involved as in the W-CDMA, the use of the power amplifier at the operating point close to saturation causes a transmit signal to distort, resulting in a wider frequency spectrum than is expected. Since in W-CDMA mobiles a transmit signal is spread at a chip rate of 3.84 Mcps, its frequency spectrum has a band of ±1.92 MHz below and above the carrier frequency fc. The spread signal is distorted in the power amplifier, spreading the spectrum up to frequencies about ±6 MHz above and below the carrier frequency fc of the transmit signal as shown in FIG. 1. If the transmit signal frequency in the W-CDMA mobile is close to 1920 MHz, the spread spectrum due to signal distortion by the power amplifier will extend to the frequency band of the PHS system, which can adversely affect the operation of the PHS system.
To avoid this, as shown in FIG. 2, one might suggest providing a guard band of a bandwidth of several megahertz in that portion of the up-link bandwidth of 1920 to 1980 MHz in the W-CDMA system which is contiguous to the frequency band assigned to the PHS system and prohibiting the use of frequencies in the guard band. In that case, however, the frequency utilization in the entire W-CDMA system would be reduced.